This invention relates to a fuel cell system having two hydrogen generating reactors.
This invention solves problems associated with the cold startup of a fuel cell system and problems associated with transient operating conditions. The following brief discussion of fuel cell system components and their operation will provide a background for understanding the problems facing the inventors of the present invention.
A primary advantage of a fuel cell is that a fuel cell can convert stored energy to electricity with about 60-70 percent efficiency, with higher efficiencies theoretically possible. Further, fuel cells produce virtually no pollution. These advantages make fuel cells particularly suitable for vehicle propulsion applications and make fuel cells a potential replacement for the internal combustion engine which operates at a less than 30 percent efficiency and can produce undesirable emissions.
A fuel cell principally operates by oxidizing an element, compound or molecule (that is, chemically combining with oxygen) to release electrical and thermal energy. Thus, fuel cells operate by the simple chemical reaction between two materials such as a fuel and an oxidant. Today, there are a variety of fuel cell operating designs that use many different fuel and oxidant combinations. However, the most common fuel/oxidant combination is hydrogen and oxygen (usually in the form of air).
In a typical fuel cell, hydrogen is burned by reacting the hydrogen with oxygen (from air) to produce water, electrical energy and heat. This is accomplished by feeding the hydrogen over a first electrode (anode), and feeding the oxygen over a second electrode (cathode). The two electrodes are separated by an electrolyte which is a material that allows charged molecules or xe2x80x9cionsxe2x80x9d to move through the electrolyte. There are several different types of electrolytes that can be utilized including the acid-type, alkaline-type, molten-carbonate-type and solid-oxide-type. The so-called PEM (proton exchange membrane) electrolyte (also known as a solid polymer electrolyte) are of the acid-type, and potentially have high-power and low-voltage, and thus are desirable for vehicle applications.
Although compressed or liquefied hydrogen could be used to operate a fuel cell in a vehicle, to date this is not practical. The use of compressed or liquefied hydrogen ignores the extensive supply infrastructure currently being used to supplying gasoline for internal combustion engine automobiles and trucks. Consequently, it is more desirable to utilize a fuel source such as methanol, gasoline, diesel, etc., to provide a hydrogen source for the fuel cell. However, the methanol, gasoline, diesel, etc., must be reformed to provide a hydrogen gas source. This is accomplished by using fuel processing equipment and hydrogen cleanup or purification equipment.
Fuel cell systems often include a fuel processing section which reforms a fuel, preferably an organic fuel such as methanol, gasoline, diesel, etc., to produce hydrogen and a variety of other byproducts. In post startup operation conditions, water and a fuel are delivered to a vaporizer and vaporized prior to charging the water/fuel vaporized stream into the reformer. However, the reforming process is endothermic and requires heat input to drive the reforming reaction. Under cold startup conditions, the reformer is cold and will not reform the fuel charged into the reformer. If steam is charged into the reformer when the reformer is cold, the steam will condense and may damage the catalyst in the steam reformer. Further, in cold weather situations, water can freeze and damage the catalyst, particularly if a pellet catalyst is used in the reformer.
The steam reformers currently being contemplated for vehicle propulsion have a substantial mass associated with the supported catalyst used to reform fuel such as methanol, gasoline, diesel, etc. Consequently, due to the large mass associated with the supported catalyst, the steam reformers take an unacceptable time to heat up to their operating temperature of about 200-300 degrees Celsius for methanol reforming, 600-800 degrees Celsius for gasoline reforming, etc. In the past, both catalytic combustors and direct fire combustors have been utilize to heat up steam reformers during cold start conditions. However, these combustors can reach very high temperature in a short period of time. This can be very problematic for methanol steam reforming which uses catalyst such as the copper-zinc that can be damaged upon exposure to elevated temperatures above 300 degrees Celsius. Thus, a quick cold startup system for methanol steam reforming has heretofore been unavailable. Other methods of quick startup would require advanced insulation technologies, and potential off-cycle pilot or electric heating.
Additional problems are associated with transient operating conditions of a fuel cell. During transient operating conditions a dramatic change in the electrical load demand on the fuel cell stack occurs. For example, a dramatic change in electrical load occurs when an operator of a vehicle attempts to accelerate the vehicle from a stopped position. The reformation process does not produce a sufficient amount of hydrogen quickly enough to meet the change in the electrical load.
Thus it would be desirable to provide a fuel cell system that avoids the problems associated with cold startup of the reformer and transient operating conditions of the fuel cell system. The present invention overcomes several of the deficiencies of the prior art.
The invention includes a fuel cell system having a methanol decomposition reactor that is used to solve cold startup and transient operating condition problems. Methanol is charged into a methanol decomposition reactor and heat is supplied to decompose methanol (an endothermic reaction) to produce hydrogen molecules and carbon monoxide. Hot exhaust gas from the methanol decomposition reactor is charged to a fuel reformer to preheat the reformer. Preheating the reformer with the hot exhaust gas from the methanol decomposition reactor brings the steam reformer up to a suitable operating temperature. Once the steam reformer is at a suitable operating temperature, methanol and water can be charged into the steam reformer without the water freezing and damaging the catalyst in the steam reformer. Further, the methanol decomposition reactor has a quick response time, relative to the steam reformer, and quickly provides a stream of hydrogen for operating the fuel cell when the system is cold.
The methanol decomposition reactor can be used to meet high load demands in transient operation of fuel cell system. Methanol can be charged to the methanol decomposition reactor when a relatively high load is required by the fuel cell stack, for example after a turndown situation. The methanol decomposition response time is relatively quick and almost instantly produces a stream of hydrogen for use by the fuel cell stack to meet a rapid change in electrical load requirements. However, because the hydrogen production efficiency of the methanol decomposition reactor is less than that of the steam reformer, it is desirable to utilize only the steam reformer after the methanol decomposition reactor has met a rapid electrical load change requirement.
In a preferred embodiment, the steam reformer can also be operated as a water gas shift reactor during cold startup conditions. Warm water, preferably from a preferential oxidation reactor downstream, can be supplied to the steam reformer to shift the equilibrium concentration of carbon monoxide in the reformer.
Another embodiment of the invention includes a fuel cell system having two hydrogen generating reactors. The first hydrogen generating reactor (e.g., a methanol decomposition reactor) is operated during cold startup conditions. A second hydrogen generating reactor (e.g., a methanol steam reforming reactor) is provided but requires heat input to reach a predetermined operating temperature above that of the cold startup temperature. During a cold startup situation, the first hydrogen generating reactor is operated to produce a hydrogen stream (A) and hot exhaust gas from the first reactor. The second hydrogen generating reactor is constructed and arranged to, after reaching a predetermined operating condition, produce a hydrogen stream for use by a fuel cell stack. The second hydrogen generating reactor is heated using the hot exhaust gas from the first reactor, while a fuel cell stack is operated utilizing the hydrogen stream (A) from the first hydrogen generating reactor. After the second hydrogen generating reactor has reached a predetermined operating temperature, it is operated to produce a hydrogen stream (B) for use by the fuel cell stack and the operation of the first hydrogen generating reactor is stopped.
Another embodiment of the invention includes a method of operating a fuel cell system during transient operating conditions wherein the electrical load demand on the fuel cell stack is dramatically increased. First and second hydrogen generating reactors are provided wherein the first hydrogen generating reactor (e.g., a methanol decomposition reactor) has a lesser hydrogen producing efficiency than that of the second hydrogen generating reactor (e.g., a methanol steam reforming reactor). The first hydrogen generating reactor also has a faster response time for producing hydrogen on demand than the second hydrogen generating reactor. Upon a dramatic increase in the load demand on the fuel cell stack, such as when an operator of a vehicle at a stopped position attempts to accelerate the vehicle, the first hydrogen generating reactor is operated to produce a hydrogen stream (A) for use by the fuel cell stack. After the dramatic increase in the electrical load demand has been met, the first hydrogen generating reactor is stopped. The second hydrogen generating reactor is operated to provide a hydrogen stream (B) for use by a fuel cell stack during ongoing normal operating conditions.
According to the present invention, heat for the endothermic decomposition of methanol may be supplied from any of a variety of heat generating components in the fuel cell system, including a catalytic combustor or a preferential oxidation reactor.
In a preferred embodiment, a catalytic combustor is utilized to provide heat to vaporize cold water and methanol during cold startup conditions. After the vaporizer has been heated by the catalytic combustor and the steam reformer has been heated by the methanol decomposition exhaust gas, methanol and water can be charged into the steam reformer. Temperature sensors may be located in the vaporizer and steam reformer to determine when it is no longer necessary to charge methanol into the methanol decomposition reactor.
These and other objects, features and advantages of the present invention will be apparent from the following brief description of the drawings, detailed description of preferred embodiments, and appended claims and drawings.